Copper is useful as a conductor material for circuit substrates intended for high density or signal propagation rate increase, because of its low cost and low resistivity and because the use of copper is free from the problem of short-circuiting or other troubles attributable to heat dissipation within the ceramic substrates that are insulating materials. In the case of using copper as a conductor material in combination with an insulating ceramic to fabricate a circuit substrate, a so-called low-temperature calcination ceramic which can be calcined at temperatures around 1,000.degree. C., such as a crystallized glass, glass-ceramic, or the like, should be used as the insulating material because of the melting point of copper.
However, since the thermal expansion coefficients of known low-temperature calcination ceramics, which range from about 1.5.times.10.sup.-6 /K to about 7.0.times.10.sup.-6 /K, differ greatly from that of copper, which is 17.times.10.sup.-6 /K, simultaneous calcination of copper and a low-temperature calcination ceramic has had the following problem. That is, even if the two materials are bonded with each other during calcination, they may separate from each other during subsequent cooling due to a difference in contraction, thereby forming a gap or crack.
As an expedient for overcoming the above problem, a technique has been proposed in which a conductor material consisting of from 95 to 98.5% by weight of conductor particles of a low-expansion material, such as WC, VC, or the like, and from 1.5 to 5% by weight of a glass is used to fill those parts of through-holes which are near the substrate surface (JP-A-3-19295, which corresponds to U.S. Pat. No. 5,029,242). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
The above technique disclosed in JP-A-3-19295, however, has a drawback in that it is troublesome that the parts of the through-holes near the substrate surface and the inner parts of the through-holes must be filled with different conductor materials. However, if all parts of each through-hole in a substrate are filled only with the above-described conductor material, too high a resistivity results.
The prior art, JP-A-3-19295, proposes to maintain an air-tight seal in glass ceramics with a use of a conductor material comprising a metals or metal carbides of, such as W, Mo, Ti, etc., and a glass, in order to overcome above drawbacks. According to the Examples of said prior art, a multi-layered substrate is sealed with an upper-most and lower-most layers which contain specific metals or metal carbides, whereas the intermediate layers are composed of the prior known substrates having through holes filled with a conductor material containing copper powder.
The specific metal or metal carbide powder of JP-A-3-19295 is required to have a thermal expansion having little higher than of the glass ceramics of about 3.0.times.10.sup.-6 /K, however, the thermal expansion of copper which is used in the present invention shows about 17.0.times.10.sup.-6 /K.
The thermal expansion coefficient of these materials are as follows.
______________________________________ Glass-ceramics 3.0 (.times. 10.sup.-6 /K) WC 3.84 TiC 7.74 VC 7.2 TaC 6.29 Mo 5.1 W 4.5 Cu 17.0 ______________________________________
Further, the use of copper as a through-hole-filling material is disclosed, for example, in JP-A-1-2864942, and has had the following problem. The conductor material for use in this application usually contains organic components, e.g., a resin, in order to enable filling the through-holes with the conductor material, and after completing the through-hole filling, the substrate is heated in the air to remove the organic components. This heating, however, causes the copper to be oxidized and thus to expand, resulting in generation of cracks in and around the through-hole conductor. In some cases, it is required to repare the cracks with a polymer, such as polyimide.